Re: kraft paper layer needed?

Er, I see. Yes it's needed, in this instance the reason for using that paper in between sand and slab is to stop dry sand to quickly absorb water from soft concrete as it gets poured on, and as it cures mainly in early stages of first 24 hours (the paper also helps just a little the sand not to be pushed by moving/spreading the concrete on top), it is important. If water was taken from fresh concrete it would not cure properly and concrete would stay week. On the other hand do not mix the concrete too wet also.

If you cannot locate this paper (probably it's a waxed paper similar to some packaging papers for packing oily items) you can use aluminum cooking foil from groceries shop.